1) Field of the Invention
The present invention relates to a technology for compensating an optical fiber transmission loss and an optical functional device loss.
2) Description of the Related Art
Optical amplifiers are used in the optical communication systems for compensating the optical fiber transmission loss and the optical function device loss. In recent years, there has been a sudden increase in the demand for the optical communications along with the spread of the Internet. As a result, the optical amplifiers have found their use even in wavelength multiplex optical communication (WDM) systems because of their wide band characteristics. Moreover, as the technology matures, the optical amplifiers are being used not only in the long-distance trunks but also in the metropolitan ring networks.
The optical amplifiers are broadly divided into rare earth-doped optical fiber amplifiers, semiconductor optical amplifiers (SOA), optical fiber Raman amplifiers and the like. The rare earths used in the rare earth-doped optical fiber amplifiers include Er (erbium) for amplifying in a band of 1525 nanometer (nm) to 1625 nm, Tm (thulium) for amplifying in a band of 1480 nm to 1510 nm, and Pr (praseodymium) for amplifying in a band of 1300 nm. Currently, the Er-doped optical fiber amplifiers (EDFA) are the main stream in the optical communication systems (for example, see Japanese Patent Application Laid-Open Publication Nos. 2000-299518, H10-262032, H11-112434, and 2000-232433).
Conventionally, control methods for the optical amplifiers include ALC control (Automatic Level Control) in which the optical output is controlled, AGC control (Automatic Gain Control) in which the gain is controlled, APC (Automatic Power Control) and ACC (Automatic Current Control) in which the excitation light or the excitation current is controlled.
FIG. 9 is a diagram to explain a general control method for an optical amplifier. A part of an input optical signal is branched by a beam splitter (BS) 901, and a photodiode (PD) 902 detects an input level of the optical signal input. One optical signal output from the BS 901 is input into an amplifying unit 903, and the amplifying unit 903 amplifies the optical signal by using excitation light of an excitation laser diode (LD) 904. On an output side of the amplifying unit 903, the optical signal is branched by a beam splitter (BS) 905, and a photodiode (PD) 906 detects an output level of the optical signal. In the AGC control, the light levels detected by the PD 902 on the input side and by the PD 906 on the output side are input into a control circuit 907 and the control circuit 907 controls excitation light of the excitation LD 904 in such a manner that the light has a predetermined and fixed gain. On the contrary, in the ALC control, the excitation light is controlled in such a manner that the light level detected by the PD 906 on the output side becomes a set light output level.
FIG. 10 is a graph to explain the amplified spontaneous emission light of an optical amplifier. When the optical amplifier amplifies a signal, it generates amplified spontaneous emission (ASE) light. The wavelength (nm) is plotted along the horizontal axis and the light output level is plotted along the vertical axis. An ASE 1001 occurs in a wavelength band wider than a wavelength band of a signal 1002. Since the PD 906 simultaneously detects both the signal 1002 and the ASE 1001, it is necessary to correct the ASE 1001 to obtain more accurately set signal gain or signal output.
In the AGC control, the correction of the ASE 1001 is carried out by two approaches. One approach is to add a corrected voltage of the ASE 1001 to a voltage output by the PD 902 to obtain a gain set in comparison with the voltage of the PD 906. Another approach is to subtract the corrected voltage of the ASE 1001 from the voltage of the PD 902 and the voltage of the PD 906 so as to obtain a set gain.
Conventionally, amplitude of the corrected voltage of the ASE is constant with respect to the set gain and output. An amount of the ASE generated greatly changes due to amplifying conditions such as the temperature of the optical amplifying unit, the power of the input signal, the power of the output signal, the power of the excitation light. As a result, although the signal gain is set to a constant value, because the ASE power is temperature dependent, the gain changes with the temperature. Similarly, the gain also changes with the power of the input signal.